getThreshold calls prepareData with resolveTree set to FALSE.

If the input tree is not binary (i.e. polytomous) then this will fail with the
following error:

    Error in prepareData(phy = phy, richness = richness, verbose = FALSE,  :
    WARNING: input tree is not binary (i.e. contains one or more polytomies).
    If you wish to randomly resolve the tree, re-run MEDUSA with option "resolveTree=TRUE".

    Stopping analysis.

The attached patch fixes this by setting the call to resolveTree in getThreshold
to TRUE by default.

I did it this way rather than changing the function signature of getThreshold to
include resolveTree because (a) it's less invasive and (b) if the user's tree is
indeed non-binary, then the call to prepareData on Line 31 of MEDUSA.R will still
raise the desired error message.